1. Field of the Invention
The invention relates to a method of combining several signals, the method being used at base stations of a radio network, carrier waves and signals being generated at the base station, the signals being used for modulating the generated carrier waves, the base station comprising an antenna for transmitting the modulated signals to a carrier wave.
The invention further relates to a base station used for combining several signals in a radio network, the base station comprising an antenna, modulating means for generating and modulating carrier waves, and transmitters for generating signals for modulating the generated carrier waves, the signals being transmitted modulated to a carrier wave using the antenna.
2. Description of the Related Art
In cellular radio systems, such as GSM/DCS systems, the base station antenna transmits several signals generated by base station transmitters. The signals modulate the carrier wave and the carrier waves are transmitted to the radio path using a radio channel. There are various methods of transmitting signals generated by transmitters to the radio path using only one antenna. It is common to use different combiners for combining several transmitters with the same antenna at the base station.
Very often a transmitting and a receiving antenna do not have a clear line of sight; instead, there are obstacles in the line of sight caused by terrain or buildings. Thus the signal perceived in the receiver is the sum of signals propagated on several routes reflected off the obstacles, and are of different phases. The sum of several signals of different phases follows the Rayleigh distribution, when the phases are randomly distributed. The fading of the signal also depends on the frequency of the signal. The frequency selective fading of a signal is one of the reasons why the frequency hopping technique is used in digital cellular radio networks. Frequency hopping means that the transmission frequency used o n a connection is changed at regular intervals. Due to frequency hopping, the transfer quality of a signal can be improved, especially in cases in which a subscriber terminal moves very slowly or is stationary, which is often the case when e.g. a mobile phone is used. In addition to the frequency diversity brought about by frequency hopping, the technique is useful in scattering the interference caused by a radio connection over several frequencies such that the momentary interference at a particular frequency remains small.
In order to achieve an advantage from frequency hopping in the digital GSM and DCS systems, more than four frequencies should be used. Currently narrowband transmitter combiners are commonly used at base stations. The narrowband combiners, whose function is to combine several transmitters with one antenna, restrict the number of frequencies used in frequency hopping to the number of actual transceiver units and combiner filters. Thus for example, a base station using three transceiver units and three combiner filters enables the use of three frequencies in frequency hopping.
Narrowband transmitter combiners are large and complex components because they have to able to be tuned. Wideband combiners have also been developed that do not require tuning such as a narrowband transmitter. Wideband combiners, together with frequency hopping synthesizers, do not restrict the number of frequencies used. A problem with wideband combiners is, however, that combiner attenuations become extremely large compared with narrowband combiners, when more than two transmitters are combined with the same wideband combiner. Hence, it has not been possible to use wideband combiners at base stations, from which a large traffic capacity is required, but narrowband combiners have had to be used. The implementation of frequency hopping at these base stations has thus been inadequate.
Alternatively, it has also been possible to combine the modulated carrier waves before the final amplification of the signal. A linear amplifier in particular has been used as the amplifier. A disadvantage of the linear amplifier has, however, been its difficult technical implementation, as the linearity requirements of the amplifier have been high. Furthermore, the performance of linear amplifiers has been poor.